It's a gallon of water.
In America just like most of the world has bottled water.
With a jug of purified water you get a lot more for a lot less We spend about 25 cents to fill a jug in Florida compared to $3 for 12 water bottles and those jugs of water are used as a substitute for bottled water as it's the same thing except more efficient as you could easily reuse the jug. It's usually used as a component in survival kits. It's easier to carry 1 jug rather than 12 water bottles.
Is that enough information? I hope that you understand the concept of it now.